1. Field of the Invention
The invention relates to a device for changing the rolling mandrels of a plug mill.
2. Description of the Prior Art
When rolling seamless steel tubes in cross-rolling and longitudinally-rolling mills, internal tools of various types are commonly use. Such tools are, for example, piercing mandrels in piercing cross-rolling mills, smoothing stoppers in smoothing cross-rolling mills and stoppers in plug or Stiefel rolling mills. It is common to all mills that the tools are large and heavy in the case of larger tube diameters and an exchange by hand --for example after each pass in the case of the plug-rolling method --means heavy physical work, as a result of which mechanically operating changing devices are used. Such a device for plug-rolling stands is described, for example, in DE-C 27 59 919. Here, a set of two or more stoppers are inserted into a change gear which has a series of pockets. For each new pass, the gear is rotated further by one pocket division, and a new plug is thereby brought into the working position. The used plug slides via a channel into the lower part of the change gear, which lies under water.
This type of change is no longer adequate for present day requirements placed on the rolling method with reference to surface quality and dimensional stability of the tubes. The point is that a change gear of the design described does not allow additional treatment steps at the stopper. Thus, for example, DE-OS 27 01 824 describes a plug-rolling method having a cylindrical rolling mandrel which can be displaced during rolling. Instead of the customary short stopper, a rolling mandrel of greater length is used here, the length being prescribed by the length of travel. As far as is known, this method is not used in practice.
DE-OS 42 13 276 describes a plug-rolling method for rolling tubes of equal wall thickness. With this method it is possible to dispense with the otherwise necessary smoothing rolling mills. In this case, specific, varying setting of the rollers in the successive passes results in a particularly uniform wall thickness. Modern methods for local measurement of hot wall thicknesses, installed directly downstream of the rolling stand, permit an instantaneous inference on the wall thickness distribution in the circumferential direction of the tube just rolled. In order to utilize this advantage, flexible adjustment of the rollers is a great advantage.
As a result of newly developed lubricants, the method according to DE-OS 27 01 824 is becoming topical again. Specifically, a cylindrical rolling mandrel, which can be displaced during rolling, does not develop its full effect until it is itself well lubricated and --as described in EP-A1 03 35 079 --the tube blank is internally lubricated and deoxidized. It is also advantageous to lubricate the hollow block internally and deoxidize it before the first stopper pass. This prevents the damaging influence of the scale, which can collect on the stoppers or rolling mandrels and lead to longitudinal grooves in the tube.
According to current points of view, it is, furthermore, undesirable for conventional stoppers or for rolling mandrels if they fall into a water bath immediately after the rolling. Since, by contrast with the core material, the surface is heated substantially more strongly, strong stresses are formed in the material and lead, in the case of abrupt cooling, to thermal shock cracks on the surface. On the other hand, however, it is necessary to use water in order to split off any scale adhering to the surface.
The fact also has to be considered that during plug rolling generally only one stopper set is used for the work, that is to say two stoppers in the case of two-pass operation. This guarantees that the tube wall thickness is not influenced by differing stopper diameters, such as can arise from differing wear, when more than one stopper set is used. It is possible to depart from this rule when devices are available for measuring the hot local wall thickness downstream of the plug-rolling stand and there is a possibility for individual setting of the rollers after each pass.
When rolling using displaceable rolling mandrels and only one set of rolling mandrels, a problem which arises is the relatively short cycle time which is available for a circulation of the rolling mandrels. In the medium tube diameter range, plug mills roll as many as 120 tubes per hour at a cycle time of 30 s. In this time, the rolling mandrel must pass through the entire manipulation chain from ejection up to reinsertion. In practice, this excludes a circulation using only one rolling mandrel set. For this reason alone, it is sensible to work using more than only one set of rolling mandrels, for example two or more sets, when the boundary conditions, that is to say local measurement of the wall thickness downstream of the plug-rolling stand, or diameter sorting of the rolling mandrels are observed. If, however, a relatively large number of rolling mandrels pass successively through the changing device, the treatment times such as, for example, cooling, remain equally short, since each rolling mandrel must be passed on by one step in the cycle of the plug mill. It is therefore advantageous not to permit all the rolling mandrel sets to pass through the same stations successively, but in the case, for example, of two or more sets to send a proportion of the rolling mandrels into a second, parallel circulation after ejection from the rolling stand. This lengthens the cycle time by the factor of the respective number of the individual machining stations. This would have the additional advantage that it is possible to operate optionally using one or two rolling mandrel series if circumstances require. In this case, a rolling mandrel series is the number of the rolling mandrels which are in circulation in order to cover all the machining stations: one series consists of a plurality of --but at least two --rolling mandrel sets.
The following requirements therefore exist for a modern plug mill:
It must be possible to use displaceable stoppers, that is to say rolling mandrels. PA1 Manual changing of the rolling mandrels is to be excluded. PA1 The rolling mandrels are to be covered before each pass with a viscous lubricant. PA1 The temperature of the rolling mandrels must be in a range which guarantees rapid drying of the lubricant without the formation of steam bubbles. PA1 The cooling of the rolling mandrels after rolling must proceed so gently that thermal shock cracks are reduced. PA1 The scale-splitting effect of the water must be retained. PA1 It must also be possible to work using a single rolling mandrel series; if the cycle time requires, using two or more rolling mandrel series. PA1 1. A waiting position for the rolling mandrel upstream of the feeding channel to the plug-rolling stand. PA1 2. An entry station for the rolling mandrels in the feeding channel of the plug-rolling stand. PA1 3. A collecting station for the used rolling mandrels downstream of the rolling, and a transfer device into the cooling device. PA1 4. A cooling station for controlled cooling of the rolling mandrels, preferably by means of a water/air mixture, as well as, at the same point, devices for removing the scale by joint application of water and mechanical aids. PA1 5. A station for drying the cooling water from the surface of the rolling mandrels. PA1 6. An intermediate station for monitoring the dimensions and surfaces of the rolling mandrels. PA1 7. A station for wetting the surface of the mandrels with a liquid lubricant which is dried by the intrinsic heat of the mandrel or by additional means such as hot air or infrared radiation. PA1 8. A depositing facility for used rolling mandrels. PA1 9A storage device for new rolling mandrels with the possibility of preheating. The device can be designed for a single series of two or more rolling mandrel sets, or for the use of two rolling mandrel series, but operated with only one series.